1. Field of the Invention
The present invention concerns the field of power electronics. It relates to a process for adjusting the carrier lifetime in a semiconductor component by introducing at least two defect regions with particle irradiation, and to means for implementing the process by using a mask to separate a beam of particles having substantially the same energy into a beam having at least two distinguishable energy groups.
2. Discussion of Background
The dynamic response of a semiconductor component is essentially influenced by its carrier lifetime. By adjusting the carrier lifetime it is possible, for example, to control on-state power losses, optimize off-state behavior and improve turn-off characteristics. The carrier lifetime is adjusted by producing defects in the semiconductor component. Such defects can basically be created in three ways: by irradiation with electrons, by doping with heavy metals, in particular with platinum or gold, or by particle irradiation with energetic heavy particles or ions, for example with protons or helium nuclei.
Irradiation with electrons leads to the defects being produced throughout the volume of the semiconductor substrate. Conversely, in the case of doping or particle irradiation, the semiconductor component is provided with defects only in a limited region, the rest of the component remaining virtually unaffected. This allows more controlled adjustment of the carrier lifetime. However, since doping processes are relatively expensive, particle irradiation has now become established as a technique for adjusting the carrier lifetime.
During this particle irradiation, a particle beam is directed at a surface of the semiconductor component, so that the particles penetrate into the semiconductor material and produce defects there. The density of the defects along the trajectory of the particles is small at first and increases steeply shortly before reaching the maximum penetration depth. The majority of the defects therefore lie in this area. The corresponding "average" penetration depth of the particles depends in this case on their energy and the composition of the matter to be penetrated. By selecting the particle energy, it is possible to determine their penetration depth in advance and therefore accurately adjust the position of the defect region. Since all the particles in the particle beam have the same energy, one particle irradiation operation produces a single defect region.
For many applications, however, a single particle irradiation operation is not sufficient to obtain an optimum turn-off characteristic. If a single defect region locally limited in a relatively sharp way is present, the current value in the turn-off process changes abruptly, overvoltages are induced and the voltage value starts to oscillate. It has, however, been shown that merely combining electron irradiation with particle irradiation leads to an improvement.
Further, it is disclosed by Solid State Electronics, Vol. 36, No. 2, pp. 133-141, 1993, that the turn-off process of a GTO (Gate Turn Off) thyristor can be improved if defect regions are present at a variety of penetration depths. To that end, the GTO is irradiated with particles several times, the individual irradiation operations being carried out with different energy values. This process does actually produce good results. However, multiple irradiation is expensive and increases the production times.